mm/submm VLBI of Sagittarius A*: An Event Horizon Telescope MBH – paradigm, useful but dangerous, need to really push techniques to make MBH case solid. Shep Doeleman MIT Haystack Observatory

Centaurus A: Optical D=3Mpc, Elliptical eating a smaller spiral galaxy, star formation along dust lanes, dust lane disk offset from radio jet. Nearest extragalactic AGN

Centaurus A: Radio

Chandra, VLA, VLT

Accretion Radiation Outflow GR in strong field regime Accretion/jet models Black hole parameters A. Fabian Chandra Web site

Vstars ~104 km/s Central Mass M ~ 4x106 M Within 45 AU. Rsch = 10as

Proper Motion Studies Is SgrA* the central mass? Luminosity consistent with stellar source. Proper motion of SgrA* measured with VLA and VLBA : +/- 1km/s upper limits (Reid & Brunthaler 2004, Backer et al 1999) If SgrA* in dynamical equilibrium with stars, then M>4x105 Msol. So SgrA* is not Stellar and is coincident with dynamical center of Galaxy.

X-ray/NIR Flares: An Indirect Size 10000 30000 20000 Time offset (s) Baganoff et al 2001 VLT: Genzel et al 2003 ~17 min periodicity? Orbiting hot-spot at R=4 Rsch Rise time < 300s Light crossing < 12 Rsch

High Frequency VLBI Resolution: /D (cm) ~ 0.5 mas /D (1.3mm) ~ 30 as /D (0.8mm) ~ 20 as ISM Scattering: scat ~  Self Absorption: Intrinsic SgrA* size decreases with 

VLBI Basics Earth Rotation Baseline Coverage Interferometer F T Fourier Components (visibilities) Map

3mm and 7mm VLBI 7mm (Bower et al 2004): VLBA Scattering size 670as, Observed size 712as Intrinsic size = 24 Rsch 3mm (Shen et al 2005): VLBA Scattering size 170as, Observed size 210as Intrinsic size = 12.6 Rsch Both in scattering dominated regime and insensitive to ISCO scale (60as for a=0). Can’t use ‘facility’ VLBI instruments VLBA: ad hoc arrays using (smaller) mm/submm apertures. High Frequency Challenges: Atmosphere, LO’s, weather, frequency reference stability. Increased sensitivity is critical.

Resolving Rsch-scale structures Falcke Melia Agol Spinning (a=1) Non-spinning (a=0) SgrA* has the largest apparent Schwarzschild radius of any BH candidate. Rsch = 10as Shadow = 5.2 Rsch (non-spinning) = 4.5 Rsch (maximally spinning)

New 4Gb/s VLBI System Total cost $40-50K per station. Digital Backend (DBE) Digital Recorder (Mark5) Total cost $40-50K per station. x16 in BW over current VLBA sustainable rates. Equivalent to replacing VLBA with 50m antennas. Planned VLBA/HSA 4Gb/s upgrade by early 2009: x4 in sensitivity over current VLBA sustainable rate.

1.3mm Observations of SgrA* 908km 4030km 4630km Carried out 230GHz VLBI. 2. High resolution. 3. See through scattering 4. Design of experiment simple (3 baselines) and aimed at determining if there was structure on scales implicated by all light curve, vlbi, proper motion evidence already shown. These high freq obs make use of non-vlbi-facility instruments: have to create arrays using separate mm/submm facilities, but it can be done. DAYS OBSERVING - April 2007

mm/submm VLBI Collaboration MIT Haystack: Alan Rogers, Alan Whitney, Mike Titus, Dan Smythe, Brian Corey, Roger Cappallo, Vincent Fish U. Arizona Steward Obs: Lucy Ziurys, Robert Freund CARMA: Dick Plambeck, Douglas Bock, Geoff Bower Harvard Smithsonian CfA: Jonathan Weintroub, Jim Moran, Ken Young, Dan Marrone, David Phillips, Ed Mattison, Bob Vessot, Irwin Shapiro, Mark Gurwell, Ray Blundell, Bob Wilson James Clerk Maxwell Telescope: Remo Tilanus, Per Friberg UC Berkeley SSL: Dan Werthimer Caltech Submillimeter Observatory: Richard Chamberlain MPIfR: Thomas Krichbaum ASIAA: Makoto Inoue, Paul Ho

Determining the size of SgrA* OBS = 43as (+14, -8) SMT-CARMA INT = 37as (+16, -10) JCMT-CARMA SMT-JCMT 1 Rsch = 10as Doeleman et al 2008

Alternatives to a MBH Most condensations of smaller mass objects evaporate on short timescales. Remaining possibility: Boson Star R=Rsch + epsilon Depends on Boson mass M87? Mass BH = 3.2x109Msol Distance = 16Mpc Rsch=4 micro arc sec. m87 is largest MBH candidate, but enclosed mass poorly constrained. Maoz 1998

A Surface or Event Horizon Does SgrA* have a surface? A quiescent surface would radiate. The radius R and dM/dt set the expected NIR flux. NIR limits set dM/dt limits. These are too low to power SgrA*. If no surface then Event Horizon. Broderick & Narayan 2006

The minimum apparent size. Broderick & Loeb Event Horizon Noble & Gammie

Caveat: Very Interesting Structures SMT-CARMA 14 Rsch (140as) JCMT-CARMA SMT-JCMT Gammie et al

But…April 2009 SgrA* Detections

SgrA*: Luminosity Leddington ~ 3.3x1044 erg/s Accretion estimates : 1.6x10-5 Msol/yr to 3x10-6 Msol/yr, stellar loss ~3x10-3 Msol/yr If 10% efficiency then Lacc ~ 1041 erg/s Lx (Chandra) = 2x1033 erg/s (2-10 keV) Lradio < 1036 erg/s SgrA* is 10-8 Ledd and over 10-5 less than expected from accretion arguments. RIAFs : Radiatively Inefficient Accretion Flows

Model Correlated Flux Density Quiescent disk models Courtesy Gammie et al and Broderick&Loeb

Constraining RIAF Models with VLBI Broderick, Fish, Doeleman & Loeb (2009)

Adding Telescopes Hawaii, CARMA, SMT Adding LMT Adding ALMA

Time Variable Structures Variabilty in NIR, x-ray, submm, radio. Probe of metrics near BH, and of BH spin. Violates Earth Rotation aperture synthesis. Use ‘good’ closure observables to probe structure as function of time.

Hot Spot Model for SgrA* Flares

Hot Spot Models (P=27min) 230 GHz, ISM scattered Models: Broderick & Loeb Spin=0, orbit = ISCO Spin=0.9, orbit = 2.5xISCO

Hot Spot Model (a=0, i=30) SMTO-Hawaii-CARMA, 8Gb/s, 230GHz, 10sec points

Closure Phases: Hawaii-CARMA-Chile Spin = 0.9 Hot-spot at ~ 6Rg Period = 27 min.

Detecting Rsch Polarization Structures

The Event Horizon Telescope An international collaborative project assemble a global submm-VLBI array for observing and resolving Event Horizons. Key observations have removed scientific uncertainty (i.e. will we see anything?). Technical Elements: low risk extensions of ongoing efforts and leverage investments from ALMA and other development projects. Collaboration: necessarily a international project but with well defined boundaries. Strong element of training at intersection of submm science and interferometry.

Event Horizon Telescope Phase 1: 7 Telescopes Phase 2: 10 Telescopes Phase 3: 13 Telescopes

Progression to an Image: 345GHz GR Model 7 Stations 13 Stations

Technical Elements of EHT Adding Telescopes: uv coverage, flare coverage, closure quantities for real-time modeling. ALMA quality, dual pol rx and LO for all sites. Testing hardware for in-situ verification. Central correlation facility. VLBI backends/recorders at rates up to 64Gb/s Phased Array processors (ALMA, PdeBure, CARMA, Hawaii) Low noise freq. references. Logistics/Observations/Project Management

Hawaii Phased Array Success (CSO+JCMT+SMA)-CARMA

EHT Phases: Phase I: 7 station 8Gb/s array ALMA phasing, preliminary Rx/LO work, new frequency standards, new site studies, operations. 2010 -- 2014 Phase II: 10 station 32Gb/s dual-pol array Activate SEST, equip S.Pole, new frequency standards, install new 0.8/1.3mm dual-pol Rx, increase bandwidth of VLBI backends/recorders, relocate ATF dishes, operations. 2015 -- 2018 Phase III: 12 station array up to 64Gb/s install ATF dishes, array operations for 5 years. 2019 -- 2024

VLBA Movie of M87 @ 43 GHz (7 mm) Walker et al. 2008 More luminous class of AGN with more massive central BH Eg M87, half the apparent size of SgrA* (1000 x more massive) Beam: 0.43x0.21 mas 0.2mas = 0.016pc = 60Rs 1mas/yr = 0.25c

April 2009 M87 1.3mm VLBI Detections CARMA1-CARMA2 CARMA-ARO/SMT JCMT-CARMA JCMT-ARO/SMT Resolved component is > 300as Compact component is 35as FWHM, or ~ 4.5 Rsch.

A Closeup of M87 Mass of M87 Black Hole = 6.4x109 Msol The angular size of the Schwarzschild radius is similar to SgrA*’s: 8as. Apparent size of Black Hole ‘Shadow’ is 40as (5.2 Rsch for non spinning BH). Apparent Size of the Innermost Stable Circular Orbit (ISCO) is 60as. These are the relevant size scales for models of TeV photon generation and jet formation. 1.3mm VLBI is now detecting compact structure on these size scales. With higher sensitivity, more VLBI sites, and the phased array at SMA, this technique can image a radio loud AGN on Rsch scales.

Summary Non-Imaging VLBI can extract BH parameters. 1.3mm VLBI confirms ~4Rsch diameter for SgrA* Non-Imaging VLBI can extract BH parameters. submm VLBI is able to directly probe Event Horizon scales and trace time variable structure (complements GRAVITY, IXO). Over the next decade: A VLBI Event Horizon Telescope will address fundamental questions of BH physics and space-time.

A Unique Opportunity “Right object, right technique, right time”

ISCO and Black Hole Spin Orbital Period gives spin. Other (indirect) methods for detection of spin: -- QPO’s -- Modeling thin disks -- GR distortions of Fe lines -- IXO: spectral obs of individual hot-spots. -- GRAVITY: NIR obs of SgrA* centroid motion. C. Reynolds